Piezoelectric actuators have many advantages not obtainable with magnetically and hydraulically operated actuators for controlling relays, valves and the like. They are small, energy efficient, have no great dissipation problem, and have quicker response time compared to devices relying on massive magnetic armatures or hydraulic, pneumatic or other mechanical devices. They are inherently less expensive and resistant to shock and vibration. Since they can be operated on very short voltage pulses, they are readily compatible with conventional logic circuits and contemporary integrated solid state fabrication methods. Further, since they can be made bistable without complex mechanical latching or double actuator coils or drives, they offer a fail-safe switching memory.
Nevertheless, piezoelectric actuators have not met with great success, for while a piezoelectric crystal will exert a considerable force against an enclosure, its motion is quite small and normally insufficient to operate a switch or valve. Schemes to overcome this limited motion problem include using the piezoelectric member to squeeze a drop of mercury through a constriction between two central chambers and employing the piezoelectric means to make a bending element, usually by mounting the piezoelectric means on a metal surface. The structure can be made with one or more piezoelectric layers and are known variously as bi-morphs, poly-morphs, bi-lams, and more generally as bending elements. Such bending elements have sufficient motion to operate switch contacts, but the closing contact force is poor. This is so because the force decreases to zero at the end of the stroke. If the contacts are set to meet before the end of the stroke then a significant force is present. However, when the piezoelectric bending element is stopped before the end of its travel it stalls: produces a back EMF corresponding to the opposing force and the contact force drops off. The applied voltage must therefore be increased to maintain the force. Unfortunately, the level of the increased voltage often is sufficient to depolarize the portion of the element which is oppositely polarized.
Among the attempts to overcome this aspect of the problem is an approach in which only one piezoelectric member is used so that there are no other oppositely polarized members which can be depolarized. Thus the element can be subjected to whatever level of voltage is necessary to overcome the back EMF and use the higher force available at midstroke. Further a snap-action or over-center spring is used to apply a negative force, push with the piezoelectric bending element, to help it overcome the back EMF. This requires a prolonged drive voltage to oppose the back EMF and is not fail-safe or bistable, as the element returns when the voltage is removed.